1. Technical Field
The present invention relates generally to medical imaging and, more particularly, to single photon emission computed tomographic (SPECT) imagers which detect gamma ray emissions from radionuclides administered to a patient.
2. Background Information
The field of medical imaging includes use of photon detectors commonly known as "gamma cameras" to detect, record, and display the uptake and distribution of radioactive drugs administered orally or intravenously to a patient. Signals generated by the gamma cameras are processed to provide a display of the internal distribution of the drugs in the patient or a particular target area or organ of the patient. The images are then interpreted by a specialist in radiology, cardiology or other relevant fields, in diagnosing any of a variety of medical conditions. Although stationary and planar gamma imaging have been used, a more advanced method is single photon emission computed tomography (SPECT) wherein a series of images or readings of gamma ray emissions are taken at regular angles around the patient, e.g., every 3 degrees about an axis through a target area or organ (such as the heart). The resulting images or data are processed using standard "back-projection" computer algorithms to construct one or more cross-sectional (tomographic) views of the body, target area, or organ. These images may be displayed as a single slice, a stack of slices oriented orthogonal to the axis or by other appropriate means. In the case of cardiac SPECT, the tomographic images are displayed as slices orthogonal to the long axis of the left ventricle.
SPECT has an advantage over conventional planar imaging of achieving greater contrast between normal and abnormal radioisotope uptake. In the diagnosis of coronary artery heart disease, increased image contrast is important to more clearly identify zones of the heart muscle (myocardium) which may be injured (infarcted) or may be at risk for subsequent ischemic injury.
Such conditions may warrant emergency admission to the hospital for more intensive observation or therapeutic intervention such as administration of anti-clotting drugs, cardiac catheterization or more radical surgery. Radionuclide SPECT has thus become a standard technique used in the diagnosis and prognosis of heart disease as well as disorders of the brain and other organs.
It is known that cardiac SPECT can be performed by rotating the gamma camera detector head through only 180 degrees rather than through a full 360 degrees. In this case, the imaging detector acquires data while it rotates through 180 degrees over the front of the chest. Because of the normal location of the heart, data recording begins with the detector head in the 45 degree right-anterior-oblique (RAO) position, which is slightly in front of the right shoulder, and ends in left-posterior-oblique (LPO) position, which is slightly behind the left shoulder.
Accordingly, manufacturers of commercial gamma cameras allow rotation of their detector heads either through 180 or 360 degrees. In addition, some manufacturers offer the choice of rotations that use an elliptical rather than circular path or orbit. In theory, an elliptical orbit allows the detector to travel in an orbit more closely corresponding to the elongate cross-section of the human torso. Accordingly, the imager head is closer to the patient's heart at all times, and provides better image quality. Additionally, some manufacturers offer the option of a continuous mode of data recording during rotation rather than the more usual "step-and-shoot" method of obtaining individual images at approximately 3 degree intervals through 180 or 360 degrees. The continuous mode reduces acquisition time, which may be important in studying very ill patients in an emergency setting.
As an alternative to rotating the gamma camera detector head around the patient, as used in conventional SPECT imaging, Kan et al., published a report in the Journal of Nuclear Medicine in the late 1970's in which they demonstrated that the detector could remain stationary while the patient was turned in a chair in front of the camera. This "rotating chair" method is currently embodied in a gamma camera manufactured by Picker International (formerly known as the Sinticor model SIM 400 Multi-crystal Camera). While rotating chair tomography may be useful with a reasonably healthy and cooperative patient, it is difficult with an acutely ill individual who must raise and maintain his or her left arm in a position above the chest and away from the heart. In patients confined to a bed this technique is unsuitable.
There are several gamma camera designs that utilize a single ring or multiple contiguous rings of radiation detectors. However, these imaging cameras are generally intended for brain SPECT and require that the patient lie on a bed or sit in a chair while the ring(s) completely surround the body or organ to be imaged. Therefore, complete ring detector configurations are impractical for SPECT imaging in an acute care setting. Furthermore, they are very heavy instruments and generally require permanent placement in a fixed location.
SPECT gamma cameras typically weigh several thousand pounds. Consequently, it is impractical to transport these imaging devices to desirable locations in the hospital such as the coronary care unit (CCU) or emergency room (ER) or department where care management decisions may depend on timely information concerning the patient's condition. One example is determining the state of myocardial perfusion in patients with unexplained chest pain, identifying risks of impending worsening of the disease and deciding if it is safe to send them home. Furthermore, because of their weight and gantry configuration, there are no known gamma cameras designed or capable of performing true SPECT imaging on patients who should remain in their existing beds, e.g., in the CCU or ER.
There are some mobile single head gamma cameras available that are capable of being transported to patient locations such as the CCU or ER. However, these instruments are not capable of performing true (multiple angle) SPECT. At best, they are capable only of performing a less useful technique called "limited angle" tomography. Examples include: "7-pinhole" and "rotating slant-angle collimator" tomography.
Consequently, the dominant method currently available to perform SPECT imaging on a patient in a CCU bed or in a similar emergency setting is to first transfer the patient to the location of the camera and place the patient on a special low attenuation bed that by design is an integral part of a gamma camera gantry assembly.
It is therefore advantageous that a SPECT imager and method be provided which allow imaging in a preexisting patient environment with little physical disturbance of the patient. Such a system would preferably accommodate multiple patient environments and orientations including supine patients in conventional hospital beds or seated patients.